I. Field of the Invention
This invention relates to an improved process for manufacturing magnets and, in particular, to a process for manufacturing ferrite magnets using coarse materials.
II. Description of the Prior Art
Typically, permanent ferrite magnets of the type MO.multidot.6Fe.sub.2 O.sub.3 (M is typically barium, strontium or lead) are manufactured by dispersing magnetic particles in a supporting matrix and subjecting the magnetic particles to an orienting magnetic field which applies a torque to the magnetic particles causing their domains to align.
Magnetic particles consist of domains in which the electrons in the outermost shell have uncompensated spins along a common axis. In a given domain the electron spins are all aligned in one direction. In a given magnetic particle, multiple domains tend to arrange themselves so that the uncompensated spins from one domain cancel those from another domain, leaving the particle in a minimum energy state and without magnetic properties. As particle size is reduced, the number of domains possible is also reduced, thus leaving the given particle with a net uncompensated spin and magnetic properties.
Exposing magnetic particles to an orienting magnetic field causes the particles to assume a specific alignment with respect to the orienting field. Accordingly, permanent magnets have previously been formed by milling magnetic materials to a very fine particle size in order to maximize the net effect of uncompensated spins, forming the particles into a generally desired shape while subjecting them to an orienting magnetic field, heating the mixture to sinter the particles, and thereafter grinding the sintered magnet to its desired final proportions.
More specifically, the standard process as previously known comprises blending the raw materials with additives and calcining the mixture. This is followed by dry and/or wet milling to a very fine particle size of about. 0.7 to 1.2 microns. This mixture is then placed in a form, subjected to an orienting field, and sintered at a temperature below the melting point until a single unified structure is formed. The magnet so formed is then cooled and machined to its final dimensions.
The many disadvantages of the standard process described above are known to those in the art. For example, with certain forming processes, such as dry pressing, extruding, and isostatic forming, the strength of the orienting field must be so great that it is difficult to fully orient the particles formed by these processes. Since the torque exerted by the orienting field increases as the cube of the particle diameter, finely ground particles require much larger orienting fields than coarsely ground particles.